The invention relates to a matrix liquid crystal display containing
two plane parallel support plates which together with a frame form a cell,
integrated non-linear elements for switching individual picture elements on the support plates and
a nematic liquid crystal mixture which is present in the cell and has a positive dielectric anisotropy and high resistivity, the liquid crystal mixture being based on the following components:
a) at least 10% by weight of a liquid-crystalline component B comprising one or more compounds having a dielectric anisotropy of more than +1.5,
b) up to 90% by weight of a liquid-crystalline component A comprising one or more compounds having a dielectric anisotropy of xe2x88x921.5 to +1.5 of the general formula I 
xe2x80x83in which
R1 and R2 are each, independently of one another, n-alkyl, xcfx89-fluoroalkyl or n-alkenyl having up to 9 carbon-atoms,
xe2x80x83the rings A1, A2 and A3 are each, independently of one another, 1,4-phenylene, 2- or 3-fluoro-1,4-phenylene, trans-1,4-cyclohexylene or 1,4-cyclohexenylene,
Z1 and Z2 are each, independently of one another, xe2x80x94CH2CH2xe2x80x94 or a single bond, and
m is 0, 1 or 2, and
c) 0 to 20% by weight of a liquid-crystalline component C comprising one or more compounds having a dielectric anisotropy of less than xe2x88x921.5, and the nematic liquid crystal mixture having a nematic phase range of at least 60xc2x0 C., a maximum viscosity at 20xc2x0 C. of 30 mpaxc2x7s and a mean dielectricity constant ∈xe2x89xa68.
Matrix liquid crystal displays (MLC displays) according to the preamble are known. For example, active elements (i.e. transistors) can be used as non-linear elements for the individual switching of the individual picture elements. This is referred to as an xe2x80x9cactive matrixxe2x80x9d, in which two types can be distinguished:
1. MOS (metal oxide semiconductor) transistors on a silicon wafer as the substrate.
2. Thin film transistors (TFT) on a glass plate as the substrate.
In the case of type 1, dynamic scattering or the guest/host effect is usually used as the electrooptical effect. The use of single-crystal silicon as the substrate material limits the size of the display, since, even if different partial displays are put together in the form of modules, difficulties arise at the joints.
In the case of the more promising, type 2, which is preferred, the TN effect is usually used as the electrooptical effect. Two technologies are distinguished: TFTs consisting of compound semiconductors, such as, for example, CdSe, or TFTs based on polycrystalline or amorphous silicon. The latter technology is the subject of intense development work worldwide.
The TFT matrix is disposed on the inside surface of one of the glass plates of the display, while the other glass plate carries the transparent counter electrode on its inside surface. Compared with the size of the picture element electrode, the TFT is very small and essentially does not interfere with the picture. This technology can also be extended to picture displays in fully satisfactory colours by arranging a mosaic of red, green and blue filters in such a manner that each filter element is opposite to a switchable picture element.
The TFT displays usually operate as TN cells which contain crossed polarizers in transmission and are illuminated from behind.
The term MLC displays in this context comprises each matrix display which has integrated non-linear elements, i.e. apart from the active matrix also displays which contain passive elements such as varistors or diodes (MIM=metal/insulator/metal).
MLC displays of this type are in particular suitable for TV applications (e.g. portable TVs) or for highly informative displays in automobile and aircraft construction. In addition to problems regarding the angle dependency of the contrast and the switching times, difficulties in MLC displays arise from the insufficient resistivity of the liquid crystal mixtures [TOGASHI, S., SEKIGUCHI, K., TANABE, H., YAMAMOTO, E., SORIMACHI, K., TAJIMA, E., WATANABE, H., SHIMIZU, H., Proc. Eurodisplay 84, September 1984: A 210-288 Matrix LCD Controlled by Double Stage Diode Rings, p. 141 ff, Paris; STROMER, M., Proc. Eurodisplay 84, September 1984: Design of Thin Film Transistors for Matrix Addressing of Television Liquid Crystal Displays, p. 145 ff, Paris]. With decreasing resistivity, the contrast of an MLC display deteriorates. Since the resistivity of the liquid crystal mixture usually decreases by interaction with the inside surfaces of the displays over the lifetime of an MLC display, a high (initial) resistance is very important for achieving acceptable service lives.
Therefore, there is still a high demand for MLC displays which have very high resistivity in combination with a large range of operating temperatures, short switching times and low threshold voltage.
The object of the invention is to provide MLC displays which do not or only to a small extent have the above disadvantages and, at the same time, have very high resistivities.
It has now been found that this object can be achieved by using nematic liquid crystal mixtures in these display elements, which mixtures are based on the abovementioned components A, B and C, B and C, or B.
Accordingly, the invention relates to an MLC display containing
two plane parallel support plates which together with a frame form a cell,
integrated non-linear elements for switching individual picture elements on the support plates and
a nematic liquid crystal mixture which is present in the cell and has a positive dielectric anisotropy and high resistivity,
xe2x80x83the liquid crystal mixture being based on the following components:
a) at least 10% by weight of a liquid-crystalline component B comprising one or more compounds having a dielectric anisotropy of more than +1.5,
b) up to 90% by weight of a liquid-crystalline component A comprising one or more compounds having a dielectric anisotropy of xe2x88x921.5 to +1.5 of the general formula I 
xe2x80x83in which
R1 and R2 are each, independently of one another, n-alkyl, xcfx89-fluoroalkyl or n-alkenyl having up to 9 carbon atoms,
xe2x80x83the rings A1, A2 and A3 are each, independently of one another, 1,4-phenylene, 2- or 3-fluoro-1,4-phenylene, trans-1,4-cyclohexylene or 1,4-cyclohexenylene,
Z1 and Z2 are each, independently of one another, xe2x80x94CH2CH2xe2x80x94 or a single bond, and
m is 0, 1 or 2, and
c) 0 to 20% by weight of a liquid-crystalline component C comprising one or more compounds having a dielectric anisotropy of less than xe2x88x921.5,
and the nematic liquid crystal mixture having a nematic phase range of at least 60xc2x0 C., a maximum viscosity at 20xc2x0 C. of 30 mPaxc2x7s and a mean dielectricity constant ∈xe2x89xa68.
The invention also relates to the corresponding liquid crystal mixtures, in particular for use in MLC displays. However, the mixtures are also suitable for many other applications, such as, for example, TN, STN or OMI.
Nematic liquid crystal mixtures which instead of the compounds of the formula I contain analogous compounds in which one of the radicals R1 and R2 is n-alkyl and the other is n-alkoxy are known and commercially utilized in various designs. However, these liquid crystal mixtures are distinguished by values for the resistivity which are too low and are often between 5xc3x97109 and 1.1xc3x971011 xcexa9cm or less at 20xc2x0. The corresponding MLC displays have values for the resistivity which are too low for some commercial applications.
The resistivity of liquid crystal mixtures is in general high, if the dielectric anisotropy is small, since the polar components which are present in mixtures which have a high xcex94∈ have a stabilizing effect on ions and thus lead to high conductivity or low resistance. Surprisingly, it has now been found that the resistivity is particularly high, if the mean dielectricity constant ∈[=31(2∈xe2x8axa5+∈11)] is small and, at the same time, the dielectrically neutral (xcex94∈ from xe2x88x921.5 to +1.5) components of the liquid crystal mixture do not contain any functional groups such as, for example, aromatically bound alkoxy or ester functions. The dielectrically positive (xcex94∈xe2x89xa71.5) components usually carry terminal cyano groups. However, in the mixtures according to the invention, it is preferred to use, in addition to compounds which have a terminal cyano, also those which have a terminal xe2x80x94NCS, F, Cl, xe2x80x94CF3, xe2x80x94CHF2, xe2x80x94OCF3, xe2x80x94OCHF2, xe2x80x94OCF2CF2H or xe2x80x94OC2F5.
However, component B can also substantially consist only of nitrile-containing compounds, of which those compounds of the formulae IIa to IIf where X is CN are preferred. In this case, the liquid crystal mixture essentially consists of components A, B and C, or A and B.
Particular preference is given to liquid crystal mixtures which contain nitrile-containing and nitrile-free, fluorinated compounds, the latter preferably conforming to the formulae IIa to IIf in which X is F, Cl, xe2x80x94CF3, xe2x80x94CHF2, xe2x80x94OCF3, xe2x80x94OCHF2, xe2x80x94OCF2CF2H or xe2x80x94OC2F5.
The ratio of nitrile-free to nitrile-containing compounds in component B is preferably  greater than 1:1, in particular  greater than 2:1. Particularly preferred ranges are 2.5:1 to 6:1.
However, very particular preference is given to liquid crystal mixtures whose component B essentially consists of nitrile-free, fluorinated compounds. Preferably, the abovementioned preferred compounds of the formulae IIa to IIf are used.
xe2x80x98Essentiallyxe2x80x99 is understood to mean that the amount of further compounds in the corresponding component is xe2x89xa620%, in particular xe2x89xa610%.
Preference is also given to liquid crystal mixtures whose component B contains compounds whose end groups are chlorinated. Compounds of this type are known to one skilled in the art and preferably conform to the formulae IIa to IIf where X is Cl. In a particularly preferred embodiment, the mixtures contain one or more compounds of the formula IIa to IIf in which A2xe2x80x94X or A3xe2x80x94X is 
where X is CF3, xe2x80x94OCF3, xe2x80x94OCHF2 or Cl. Furthermore, component B can also contain tetranuclear compounds, for example in accordance with formulae IIc to IIf in which one of the rings A1 to A3 is present twice.
In a particularly preferred embodiment, the mixtures contain compounds which have a terminal nitrile and are present in component B in an amount from 0 to 50% by weight. Particular preference is given to mixtures which do not contain any compounds which have a terminal nitrile. Surprisingly, it has been found that groups such as xe2x80x94OCF3, xe2x80x94OCHF2, xe2x80x94OCF2CF2H or xe2x80x94OC27F5 have a considerably less stabilizing effect in the displays than xe2x80x94OCH3 or xe2x80x94OC2H. The same is true for aliphatically bound alkoxy (compounds of the formulae III and IV).
The mixtures according to the invention preferably have a resistivity of xe2x89xa71012 xcexa9xc3x97cm, particularly preferably  greater than 1013 xcexa9xc3x97cm, at 20xc2x0. The mean ∈ is preferably xe2x89xa67, in particular xe2x89xa65.
The values of dielectric anisotropy of the individual compounds of components A to C are determined at 20xc2x0 by extrapolation from a polar mixture (containing 24% of p-trans-4-propylcyclohexylbenzonitrile, 36% of p-trans-4-pentylcyclohexylbenzonitrile, 25% of p-trans-4-heptylcyclohexylbenzonitrile and 15% of 4-cyano-4xe2x80x2-(trans-4-pentylcyclohexyl)biphenyl), if the compound to be determined contains a dipole along the longitudinal axis of the molecule, or from a neutral mixture (containing 22% of trans-1-p-ethylphenyl-4-propylcyclohexane, 20% of trans-1-p-methoxyphenyl-1-propylcyclohexane, 15% of trans-1-p-ethoxyphenyl-4-propylcyclohexane, 19% of 4-ethyl-4-(trans-4-propylcyclohexyl)biphenyl, 14% of 4-ethyl-4xe2x80x2-(trans-4-pentylcyclohexyl)biphenyl, 5% of 4,4xe2x80x2-bis(trans-4-propylcyclohexyl)biphenyl and 5% of 4-(trans-4-pentylcyclohexyl)-4xe2x80x2-(trans-4-propylcyclohexyl)biphenyl) in the case of neutral compounds.
The liquid crystal mixtures according to the invention make it possible to achieve a high value for the resistivity in combination with low viscosities, which allows to produce excellent MLC displays. The MLC displays according to the invention preferably operate in the first transmission minimum according to Gooch and Tarry [C. H. Gooch and H. A. Tarry, Electron. Lett. 10, 2-4, 1974; C. H. Gooch and H. A. Tarry, Appl. Phys., Vol. 8, 1575-1584, 1975], in which case in addition to particularly favourable electrooptical properties such as, for example, high steepness of the characteristic curve and low angle dependence of the contrast (German Patent Specification 3,022,818) in combination with the same threshold voltage as in an analogous display, a smaller dielectric anisotropy is sufficient in the second minimum. This makes it possible to achieve significantly higher resistivities in the first minimum, when the mixtures according to the invention are used.
The viscosity at 20xc2x0 C. is preferably xe2x89xa625 mpaxc2x7s. The nematic phase range is preferably at least 70xc2x0, in particular at least 80xc2x0. Preferably, this range extends at least from xe2x88x9220xc2x0 to +70xc2x0.
The individual compounds of the formulae I to IV and their subformulae which can be used in the MLC displays according to the invention are either known or can be prepared analogously to known compounds.
Preferred liquid crystal mixtures to be used according to the invention contain a total of, preferably, 10% to 90%, in particular 20% to 90%, of compounds of the formula I. If component B is not composed predominantly of strongly dielectrically positive nitrile components but predominantly only of weakly dielectrically positive compounds such as, for example, the fluorinated compounds mentioned below, component A can under certain circumstances be omitted entirely, and the mixtures according to the invention can in this special embodiment be solely based on component B and, if desired, component C. Particular preference is given to liquid crystal mixtures whose component B contains one or more compounds selected from the group consisting of compounds of the formulae IIa to IIf 
in which
R is n-alkyl or n-alkenyl of up to 9 carbon atoms,
X is cyano, xe2x80x94NCS, F, Cl, xe2x80x94CF3, xe2x80x94CHF2, xe2x80x94OCF3, xe2x80x94OCHF2, xe2x80x94OCF2CF2H or xe2x80x94OC2F5, and
the rings A1, A2 and A3 are each, independently of one another, 1,4-phenylene, 2- or 3-fluoro-1,4-phenylene, 2,3-difluoro-1,4-phenylene, trans-1,4-cyclohexylene or 1,4-cyclohexenylene.
Preferably, component B contains compounds of the formulae IIa to IIf in which X is cyano and compounds of the formulae IIa to IIf in which X is xe2x80x94NCS, F, Cl, xe2x80x94CF3, xe2x80x94CHF2, xe2x80x94OCF3, xe2x80x94OCHF2, xe2x80x94OCF2CF2H or xe2x80x94OC2F5, and the amount of cyano compounds in component B is 0 to 50% by weight.
In a particularly preferred embodiment, component B does not contain any compounds of the formulae IIa to IIf in which X is cyano.
In the compounds of the partial formulae IIa to IIf, X is preferably F, Cl, CF3, xe2x80x94OCF3, xe2x80x94OCHF2 or xe2x80x94CHF2.
The rings A1, A2 and A3 are each preferably, independently of one another, trans-1,4-cyclohexylene or 1,4-phenylene. In a preferred embodiment, one of the rings A1, A2 and A3 is 2- or 3-fluoro-1,4-phenylene or 2,3-difluoro-1,4-phenylene. The ring bound to X (i.e. A2 in IIa and IIb and A3 in IIc to IIf) is preferably 1,4-phenylene which is unsubstituted or even mono- or di-substituted by fluorine. A2xe2x80x94X and A3xe2x80x94X are preferably a group selected from the formulae (a) to (h): 
of which (a), (b), (d), (f), (g) and (h) are particularly preferred.
Particularly preferred smaller groups of compounds are listed below: 
In the partial formulae IIa1 to IIf3, R is in each case n-alkyl or n-alkenyl of up to 9 carbon atoms. Y and Z are each, independently of one another; H or F although one or two of the radicals Y and Z is preferably fluorine. However, X is preferably F, Cl, xe2x80x94CF3xe2x80x94, xe2x80x94OCF3 or xe2x80x94OCHF2.
Component B preferably represents 10% to 100%, in particular 20% to 80%, of the mixtures according to the invention.
Component A preferably contains one or more compounds selected from the group consisting of II1 to II7: 
in which R1 and R2 have the meaning given in claim 1.
Preferably, component A additionally contains one or more compounds selected from the group consisting of II8 to II20: 
in which R1 and R2 have the meaning given in claim 1 and the 1,4-phenylene groups in II8 to II17 can each, independently of one another, also be mono- or polysubstituted by fluorine.
Furthermore, component A preferably additionally contains one or more compounds selected from the group consisting of II21 to II25: 
in which R1 and R2 have the meaning given in claim 1 and the 1,4-phenylene groups in II21 to II25 can each, independently of one another, also be mono- or polysubstituted by fluorine.
Finally, those mixtures are preferred whose component A contains one or more compounds selected from the group consisting of II26 and II27: 
in which CrH2r+1 is a straight-chain alkyl group of up to 7 carbon atoms.
In some cases, the addition of compounds of the formula 
in which
R1 and R2 have the meaning given in claim 1 and
Z0 is a single bond, xe2x80x94CH2CH2xe2x80x94, 
proves to be advantageous for suppressing smectic phases, although the resistivity is thereby lowered. Whether and in which amount these compounds should be added for achieving optimum parameter combinations for practical application can easily be determined by one skilled in the art. Usually, less than 15%, in particular 5-10%, are used.
Further preference is given to liquid crystal mixtures which, in addition to components A, B and C, additionally contain one or more compounds selected from the group consisting of III and IV: 
in which R1 and R2 have the abovementioned meaning and/or one or more compounds selected from the group consisting of V and VI 
in which R1 and R2 have the abovementioned meaning and/or one or more compounds selected from the group consisting of VII to XI 
in which R1 and R2 have the abovementioned meaning and s is 0 or 1.
The amounts of the compounds of the formulae III to XI in the mixtures according to the invention (preferred ranges) can be seen from the table below:
Sum of compounds III and IV: 0% to 40%, preferably 10% to 30%
Sum of compounds V and VI: 0% to 40%, preferably 5% to 20%
Sum of compounds VII to XI: 0% to 20%, preferably 5% to 15%
It goes without saying that the mixtures according to the invention, which preferably consist essentially of the compounds mentioned as preferred for components A to C, may additionally also contain further compounds not mentioned here explicitly. However, this leads in many cases to more unfavourable properties. One skilled in the art can easily determine whether and in which amounts further compounds can be used.
The design of the MLC display according to the invention which consists of polarizers, electrode base plates and electrodes which have been subjected to surface treatment is that which is customary for this type of display. The definition of customary design is in this case very broad and also comprises all modifications and alterations of the MLC display, in particular also matrix display elements based on poly-Si TFT or MIM.
However, a significant difference between the displays according to the invention and those which have been customary so far and are based on the twisted nematic cell is the selection of the liquid crystal parameters of the liquid crystal layer.
The preparation of the liquid crystal mixtures to be used according to the invention is carried out in the usual manner. As a rule, the desired amount of the components used in a minor amount is dissolved in the components which constitute the major component, advantageously at elevated temperature. It is also possible to mix solutions of the components in an organic solvent, for example in acetone, chloroform or methanol, and remove the solvent again after the mixing, for example by distillation.
The dielectrics can also contain further additives known to one skilled in the art and described in the literature. For example, 0-15% pleochroic colorants or chiral doping substances can be added.